Boreholes are formed in the subsurface of the Earth in many contexts. They provide access to the interior of the Earth's crust, as may be desirable for example to construct a well to extract fluids from geological formations in the Earth, or perhaps to explore or make measurements of the subsurface. The borehole is drilled using drilling equipment and is typically cased or lined with tubular sections of casing or lining. The casing or lining can help to support and stabilise the geological formation into which the borehole is drilled in order to prevent collapse of the formation. It may also help to prevent fluid pressure loss or build up in the borehole, which can be important for safely performing further borehole operations such as drilling.
In order to case the borehole, an initial casing (i.e. tubular) is inserted at a desired location in a drilled section of the borehole. Cement is pumped and injected into the borehole to enter the space (annulus) surrounding the inserted casing. The cement circulates up along the outside of the casing in the annulus between the casing and the formation, and is left to set and harden to secure the casing in place.
At more advanced stages, a further casing may be installed. The further casing has a smaller internal diameter and is inserted radially within the initial casing, approximately concentrically therewith forming an annular space between the inner surface of the initial casing and an outer surface of the further casing. The further casing is installed in the same way as the initial casing, with cement pumped into the borehole and forced up through the annular space between the initial and further casing.
In this way, a multi-cased region can be defined in the borehole where the borehole has a wall structure including multiple layers of casing spaced apart from each other in a radial direction with respect to the borehole long axis, toward the formation.
A difficulty with the casing process in practice is that cement may not completely or perfectly fill the annular spaces surrounding the casings. Accordingly, there may be gaps where cement has not reached, and potential pathways for fluid from the formation to leak into the borehole, or vice versa, which can create problems for pressure control in the borehole. In addition, if a borehole or well is to be abandoned the borehole is required to be plugged to prevent leakage of fluid from the formation to the surface. Cement plugs can be acceptable for this purpose, but must comply with stringent leakage and pressure containment requirements. For example, a permanent well barrier may be required to have some or all of the following properties: 1) impermeability; 2) long term integrity; 3) non-shrinking; 4) ductile (i.e. non-brittle and able to withstand mechanical loads/impacts); 5) resistance to chemicals/substances (e.g. H2S, CO2 and hydrocarbons); and 6) wetting, to ensure bonding to steel.
It is known to assess the quality of the cementation and whether the cement is adhered solidly to the surfaces of the casing in a logging operation. Sonic logging tools have been used for this purpose. A more recent technique is to obtain cement evaluation logs, which give detailed 360-degree representations of the integrity of the cementation.
In some techniques, variations in amplitude of an acoustic signal travelling in the casing wall between a transmitter and a receiver are detected and used to determine the quality of the cement bond on the exterior casing wall. The fundamental principle of this determination is that the acoustic signal is more attenuated in the presence of cement than if the casing were not cemented. This technique has limitations in that the measurement is largely qualitative, as there is no indication of azimuthal cement variations such as channelling and as it is sensitive to the effect of a micro-annulus.
Pulse echo techniques have been developed where an ultrasonic transducer, in transmit mode, emits a high-frequency acoustic pulse towards the borehole wall, where it is reflected back to the same transducer operating in receive mode. The measurement consists of the amplitude of the received signal, the time between emission and reception, and sometimes the full waveform received. Tools that use this technique either have multiple transducers, facing in different directions, or rotate the transducer while making measurements, thereby obtaining a full image of the borehole wall. In cased hole, the waveform is analysed to give indications of cement-bond quality and casing corrosion.
In addition, it is known to excite flexural waves in the casing, obtain an amplitude signal and use the attenuation of the signal to determine properties of the material, whether that be a solid (e.g. cement), liquid or gas, adjacent to the casing.
It is known to generate flexural waves of this type using an ultrasonic pulse-echo tool with a transmitter arranged to transmit a pulse obliquely incident with respect to the casing and a receiver arranged to receive reflections or echoes of the pulse from interfaces in the borehole wall.
However, existing technology is not able to quantify the quality of a cement plug in an interval between two surfaces, taking into account the bond with both surfaces simultaneously. Furthermore, existing technology does not take into account the scattering and absorption of a signal along the path in the analysed layer by means of taking into account both types of attenuation: share wave attenuation and flexural wave attenuation. In addition, the quality of a cement bond is not currently defined in terms of intrinsic properties of the material.
It is therefore an aim of the present invention to provide a method of identifying a material and/or condition of a material in a borehole that helps to address the above-mentioned deficiencies.